“I’ll tell you what we’ll do next time we hear it,” said Teddy as the boys picked up the provisions they had brought and started toward the house.  “We’ll go up on the roof.  Then we’ll pretty soon see whether it’s a ghost or the real thing.”

“And in the meantime,” suggested Chet, sniffing the air hungrily, “how about some supper?”

CHAPTER XXI

BOTH AT ONCE

It was not long before there came a recurrence of the strange humming noise which had so disturbed the girls.  It was only a few nights later that Chet sat up in bed with the joyful feeling that here at last was a chance to investigate at least one of the ghosts that haunted the homestead at Cherry Corners.

“Ferd!  Teddy!  Wake up!  What’s the matter?  Are you dead?” he called to the boys.

The latter reluctantly opened their eyes and looked at him reproachfully.

“Can’t you let a fellow sleep?” Teddy asked.  But Chet, with no ceremony whatever, hauled him bodily out of bed and set him on his feet.

“Don’t talk,” he ordered.  “Run as fast as you can to the roof before we miss it.”

“What are you raving about?” asked Ferd, although both he and Teddy started obediently toward the attic stairs.

“If you wouldn’t talk so much, you could hear it,” Chet answered, pushing up a trap door that led to a small square platform on the roof.  “It’s the motor sound the girls heard and that scared them so.”

“It is, for a fact!” cried Teddy in a joyful whisper.  “And it’s coming right near, fellows, too.”

“It’s an aeroplane all right,” said Ferd, with conviction.  “Nothing else ever made a noise like that.”

“Say, what are you doing up there?” a girl’s voice hailed them from the bottom of the steps, and Chet thought he recognized it as Billie’s.  “Are you walking in your sleep or have you gone crazy?  Come down here quick, we need you.”

“Keep still,” Chet yelled back.  “We’re looking for your aeroplane ghost.  Can’t you hear it?”

“Yes.  But, oh, Chet,” Billie’s voice was tremulous, “the piano is playing itself again.  Won’t you come down?  We’re afraid to stay here all alone.”

“Great Scott! all the spirits are roaming at once,” cried Teddy, straining his eyes to see through the darkness as the humming of the motor came nearer.

“There, isn’t that it?” cried Ferd, pointing eagerly through the trees toward a little patch of sky, palely illumined with stars.

“I think I saw it,” said Chet, rubbing his eyes impatiently.  “It’s so confoundedly dark—­”

“Oh, won’t you please come down?” wailed Billie’s voice from the spooky depths of the attic.  “I’ll die of fright if I have to stay here another minute.”

This appeal moved the boys, and they began reluctantly to descend the ladder, keeping their eyes all the time on the pale patch of sky.